Re: Tohatsu Rectifier
I suspect you may find that this charging system is unregulated. Don't confuse a rectifier (makes DC for charging) with a regulator. My old OMC 15's were also unregulated, so I'm familiar with them.
In unregulated systems, the entire system must be top notch. All components -- every cable and connection, including the battery, must be checked. If there is high internal resistance in the battery, for instance, the charge voltage will go pretty high. The unregulated OB charging systems can overcharge slightly, but they only make a few amps, so usually not a problem.
Is your voltmeter reliable, and is it connected right to the battery? Many cheap meters are off by more than 20%. You can check the actual battery voltage (flooded-cell batteries should be about 13.6v when fully charged) with a good meter. Test right at the battery, with nothing else connected, to rule out any wiring influences. If the battery is fully charged, then allowed to rest and stabilize for a while, and the battery voltage is still low, the battery has a problem. This will lead to the charging system trying to charge it too much, to make up for the problem of the battery.
The same holds true for the wiring. Just one slightly corroded connection will cause a high voltmeter reading, while the charge amps will still be within specification. This is one reason why the older, unregulated outboards were not supposed to be run without a battery connected -- the charge voltage would go so high that it would fry the rectifier diodes.
I would check the actual charging amps -- at the battery -- with a good ammeter. Most flooded-cell marine batteries will live happily with less than 5 amps going to them, even if they are fully charged, though they will gas a little. If your charge amps are low, yet the voltage is high, there is a wiring issue to be corrected. A charging voltage of 14 or 15 is not unusual in an unregulated system, but 17 and up is definitely high.
The new Tohatsu/Nissan charging systems (on the 8/9.8 and 15/20 4-strokes) are regulated, which allows them to tolerate imperfections in the boat's electrical system a little better. There may be a retrofit kit to install a regulated rectifier on your 2-stroke 18.
